The invention relates to an improved oil-base drilling fluid. The improved drilling fluid has the stability, rheological properties, and hole cleaning abilities required for drilling fluid applications; but it is less toxic than known oil-base drilling fluids and exhibits greater environmental compatibility with land disposal methods than current oil-base drilling fluids. More particularly, the improved drilling fluid incorporates novel compounds into the solution used to form the internal water phase. Also, the use of low aromatic content oils for the continuous oil phase in the preferred embodiment further reduces the toxicity and improves the environmental compatibility of the drilling fluid.
Drilling fluids or muds are an important component of petroleum exploration and production. These fluids, which are made with a variety of components, are used to clean drill bits, remove cuttings from holes, and maintain drilling pressure. The rheological properties of a drilling fluid are critical because the fluid must exhibit certain properties to accomplish these tasks and must maintain these properties during continued use at well conditions.
Drilling fluids may be either water-base or oil-base. Typically, water-base drilling fluids are used for drilling operations, but they suffer from disadvantages related to the nature of water as used in drilling applications. Specifically, water migrates from the drilling fluid into surrounding clay or shale formations and causes disintegration or alteration of the clay or shale formation. Further, the water will dissolve salts in the clay or shale formation, interfere with the flow of gas or oil through the formation, and corrode iron in the drilling equipment.
Oil-base drilling fluids, on the other hand, do not affect clay or shale formations or soluble salts in the formations, because oil is native to these formations. Further, oil-base drilling fluids provide several advantages over water-base drilling fluids such as better lubricating qualities, higher boiling points, and lower freezing points. Because oil-base drilling fluids cost more than water-base drilling fluids, they are used in applications where they provide superior performance under particular conditions.
Oil-base drilling fluids typically contain some amount of water. This water may occur in concentrations less than approximately 5 percent as an emulsified contaminant in oil-base drilling fluids. In other oil-base drilling fluids, water is intentionally added along with an effective emulsifier to produce a water-in-oil or invert emulsion. An emulsifier is necessary to prevent over thickening of the drilling fluid which typically occurs when higher concentrations (&gt;5%) of water are used in oil-base drilling fluids. These emulsions use water as a suspending agent for various components of the drilling fluid, and typically contain 10 to 60 percent water.
Oil-base invert-emulsion drilling fluids include two phases: (1) A continuous phase containing oil (typically No. 2 Diesel Fuel), surfactants, and wetting agents; and (2) a dispersed internal water phase which is often a water-based solution of calcium chloride.
The water in the internal phase of an invert emulsion drilling fluid may act just as the water in a water-based drilling fluid and migrate into surrounding clay or shale formations with negative effects on the formation. This migration is primarily due to the thermodynamic properties of the water. For example, the thermodynamic activity of a pure water internal phase in a drilling fluid is higher than the thermodynamic activity of water in clay or shale formations which contain dissolved salts. Consequently, there is a tendency for the thermodynamic activities of the water in the drilling fluid and the water in the clay or shale formation to equilibrate. This occurs by the transfer of pure water into the clay or shale formation and the associated transfer of dissolved salts into the pure water of the drilling fluid. The transfer of water from the drilling fluid to the clay or shale formation may cause the formation to swell and crack.
The thermodynamic tendency of the water in a drilling fluid to migrate into the surrounding formation can be measured as a vapor pressure, and is commonly referred to as the water activity. The water activity is referenced as the tendency that the solution will migrate relative to pure water under the same conditions. Solutions, especially chloride solutions, are used in the internal phase in known oil-base drilling fluids to minimize the water activity of the internal phase. Solutions are used instead of pure water to decrease the migration of water from the drilling fluid into surrounding formations because the dissolved salt decreases the water activity. Chloride salts such as calcium chloride are often used in known drilling fluids as the dissolved salt in the internal phase for the purpose of controlling the water activity of the internal phase.
It should be appreciated that by accurately measuring the water activity of the water in surrounding formations, the water activity of the internal phase in an invert emulsion drilling fluid may be adjusted by the proper addition of salt to match the water activity in the surrounding formation. This prevents the transfer of water between the drilling fluid and the surrounding formation, and avoids adverse effects on the surrounding formation. Generally, sufficient calcium chloride is added to balance the lowest water activity of surrounding formations and the emulsified water of the drilling fluid.
Unfortunately, calcium chloride solutions and other halide salt solutions are toxic to life, especially plant life. Problems associated with environmental contamination and oil-base drilling fluid disposal are well documented. (See, for example, George R. Gray and H. C. H. Darley, Composition and Properties of Oil Well Drilling Fluids, Fourth Edition, Gulf Publishing Company at page 585). Concern has been expressed by environmentalists and others with the possibility of polluting underground water supplies, damaging soil productivity and diminishing surface water quality. In a conference sponsored by the Environmental Protection Agency in May of 1975 in Houston, Tex., the effects of both techniques and chemicals used in drilling fluids and their impact on the environment were discussed. The outlook for landfill disposal of oil-base drilling fluids was not good. Such muds were thought to be toxic and the effects long-term. The toxic effect of oil-base muds on the soil was thought to be inherent in the chemicals used. Thus, known oil-base drilling fluids using a calcium chloride internal phase have adverse environmental consequences when used for land drilling operations.
Preferably, land farming could be used to dispose of both drilling fluids and the cuttings produced at a land drilling operation. And, the land farm would ideally be located near the site of the drilling operation. It should be appreciated that the cuttings contain an amount of drilling fluids. In land farming, the spent drilling fluids and cuttings would be spread over a section of land and plowed into the ground using standard agricultural methods. Drilling fluids using chloride solutions in their internal phases have proven too toxic to be acceptably disposed of by land farming, however.
Environmental regulations also restrict the concentration of halides, nitrates, sulfates, and phosphates in drilling fluids used for land drilling operations. Thus, there is a need for oil-base drilling fluids having a composition that will comply with environmental regulations and will be environmentally compatible with land disposal methods.
Known drilling fluid compositions have used acetate salts in low concentrations for various purposes. For example, U.S. Pat. No. 4,148,736 discloses the use of sodium acetate as a buffer salt in a water-chloroform drilling fluid for specialty applications such as tertiary oil recovery. Col. 3, lines 57-61. Similarly, U.S. Pat. No. 4,537,688 discloses the use of sodium acetate to buffer a polymerization reaction in a sulfonated terpolymer ionomer viscosification agent for drilling fluids. Col. 5, lines 64-67.
While the use of acetic acid has been noted as a vapor pressure depressant for the water phase of invert emulsion drilling fluids, U.S. Pat. No. 3,702,564, Col. 13, lines 58-62, its beneficial effect on the toxicity of invert emulsion drilling fluids was not noted. It should be appreciated that the addition of any water soluble material to a water solution will decrease the vapor pressure and water activity of the solution.